This invention relates generally to a circular traveling grate for recovering oil from oil bearing materials such as shale and, more particularly, to a traveling grate machine having a new and improved mechanism to discharge spent shale from the grates and a water-cooled frame structure. As is pointed out in U.S. Pat. No. 3,325,395, extremely large deposits of oil bearing shale are known to exist in the United States and particularly in Colorado and Wyoming. Shale rock and certain oil sands contain a substance called "kerogen" or "petrogen," which is an organic waxy compound. When heated to a temperature of about 800.degree. F, it cracks partially to yield a substance which has the properties resembling crude oil. In fact, any organic or organic containing material will yield such a substance when treated in that manner.
The United States Bureau of Mines has conducted considerable experimental work on the recovery of shale oil from these western deposits. Its studies have resulted in the shaft furnace method of recovery of the oil, whereby a quantity of oil bearing rock is deposited in a vertical furnace and submitted to an updraft of heated gasses. This technique produces relatively small amounts of oil and is somewhat inefficient due to the turbulence in the shaft furnace. Another independent investigation utilizing a retort method of recovery, which is similarly inhibited, utilizes a downdraft procedure followed by recovery of the kerogen components from the gasses.
A continuous machine for removing oil from oil shale is shown in U.S. Pat. No. 3,302,936. The machine disclosed therein is a circular traveling grate machine, having a plurality of pallets which are movable along a circular trackway, mounted on a conventionally designed superstructure. A stationary hood and wind ducts are mounted above and below the trackway. Crushed shale is brought to the machine and deposited on the pallets, and the pallets then move through successive zones where hot gasses are fed downwardly through the burden to remove the oil in the form of a mist. In a second zone the shale is cooled by forcing gas upwardly through the burden.
The pallets are then moved to a dumping zone where the spent shale is discharged. The arrangement set forth in U.S. Pat. No. 3,302,936 for dumping the pallets includes the pallets being mounted on a pair of wheels located at the leading edge of the pallet and by a third wheel adjacent its trailing edge. The two leading wheels are located on tracks positioned outside the combustion zone, while the third wheel glides along an interior track. The pallets are dumped by being pivoted on the leading wheel axle as the trailing wheel follows a downwardly dipped portion of the central trackway, which is located within the combustion zone.
While this dumping arrangement is suitable for many materials, it has been found that an oil shale environment is particularly hostile with respect to the trailing wheel, its bearing, and the internally located track. This type of a dumping procedure is known in the art as an axial dump and a further problem is involved when the pallets are dumped in this manner. The leading edge of the pallet forward of the front wheels must tilt above the normal horizontal plane of the pallet when it enters the discharge zone. Frequently, this front edge is not able to be raised by the weight of the pallet due to bridging or fusion of the material in the discharge zone. This is particularly true when the depth of the burden may be between 8 and 10 feet.